Process for affixing ohmic contacts to photoconductor elements



7, 1965 N M. BERKENBLIT ETAL 3,

PROCESS FOR AFFIXING OHMIC CONTACTS T PHOTOCONDUCTOR ELEMENTS Filed June 4, 1962 W I +1 FIG. 1 2.5 Y I .5mcl/div.

Q N V=2v/div.

-3 l I l I l l I 3 x DNSTRIBUTION CURVE PPMCu f I CURVE N0.I 4 ELECTRODES N0N-oNN|c 22 CURVE N0.]1 2o VSILVER PASTE ELECTRODES (NUMBER 15- -oNN|c OF SAMPLES NANNNcAcwEN VALUE or RESISTANCE) 6 4 x ,4 K 2 X I 0 .140 160 200 N Nun-"m v INVENTORS r MELVIN BERKENBLIT Fl G. 2 (RESISTANCE m NNoNNs) GEORGE CHEROFF FREDERICK HOCHBERG ARNOLD REISMAN BY M\W &

ATTORNEY United States Patent 3,222,216 PROCESS FOR AFFIXING OHMIC CONTACTS T0 PHOTOCONDUCTOR ELEMENTS Melvin Berkenblit, Yorktown Heights, George Cheroff,

Peekskill, and Frederick Hochberg and Arnold Reisman, Yorktown Heights, N.Y., assignors to International Business Machines Corporation, New York, N.Y., a corporation of New York Filed June 4, 1962, Ser. No. 199,799 7 Claims. (Cl. 117-212) This invention relates to a process for afiixing ohmic contacts to photoconductor elements. More particularly, it relates to afiixing ohmic contacts to cadmium selenide photoconductor elements. Prior to this time ohmic contacts could not be reproducibly afiixed to sintered layer photoconductors.

Among the critical steps involved in the preparation of reproducible sintered layer photoconductors is the surface preparation of the processed sintered layer prior to affixing the electrodes thereto. Under certain conditions of preparation, as for example, those disclosed in US. application S.N. 172,699 filed February 12, 1962, now Patent No. 3,145,120, issued August 18, 1964, entitled, Method for Controlling Flux Pressure During a Sintering Process, by George Cheroif, Frederick Hochberg, Arnold Reisman and S01 Triebwasser, it is observed that the reproducibility of the finished photoconductor layers may be greatly enhanced with incorporation of a technique for improving the ohmicity of the electrodes that are afiixed. This technique not only enhances absolute reproducibility but also yields more sensitive photoconductor elements.

The process of the invention produces an electroded sintered layer which exhibits a minimum of non-linear current-voltage behavior.

An object of the invention is to affix ohmic contacts to photoconductor elements.

Another object of the invention is to aflix silver paste electrodes to cadmium selenide photoconductor elements.

A further object of the invention is to remove the barrier layers formed during the sintering process prior to afiixing electrodes to the photoconductor element.

Still another object of the invention is to remove the cadmium oxide barrier layer prior to affixing the electrodes to the photoconductor elements with nitric acid.

A further object of the invention is the preparation of ohmic contacts of photoconductor elements so that re producible photoconductors can be prepared.

Another object of the invention is to enable scaling of electrode spacing.

Still another object of the invention is to increase the sensitivity of photoconductor elements to a lower value than is normally obtained by a given process.

An additional object of the invention is preparing the surface of a photoconductor for electrodes so that the electrodes thus aflixed are ohmic in nature.

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawing.

FIGURE 1 shows oscilliscope curves of a current (I) vs. voltage (V) plot for cadmium selenide photoconductor elements illustrating behavior with ohmic and non-ohmic electrodes. Curve A demonstrates the linear behavior 3,222,216 Patented Dec. 7, 1965 of a photoconductor element which has been subjected to the process of the invention and thus has electrodes which are ohmic in nature. Curve B is that obtained for a photoconductor element which has non-ohmic electrodes (Le. has not been subjected to the process of the invention).

FIGURE 2 compares the distribution curves of reproducibility of cadmium selenide photoconductor elements prepared according to the process of the invention (Curve II) and those elements prepared without using the process of the invention (Curve 1). It is evident from inspection of FIGURE 2 that photoconductors prepared by the process of the invention have a lower average resistance and therefore a greater sensitivity and exhibit a much smaller absolute resistance spread than those not incorporating the process of the invention.

Two serious problems attendant with the preparation of photoconductors used in machine logic technology involve the preparation of reproducible photoconductor elements and the ability to alter electrode geometry such as electrode spacing (electrode separation), and obtain scaling of electrode properties.

The phenomenon known as scaling is understood to mean that when an electrode separation is reduced to a fraction of its previous value, the light resistance of the element will be reduced by the same value. A single parameter which aifects both of these properties markedly is the ohmicity of the photoconductor-electrode junction.

It has been found that if the electrode contacts are nonohmic, namely if the current-voltage characteristics are non-linear, the resultant finished elements are not as reproducible as desired, and in addition one cannot scale electrical properties when electrode gaps are varied. Possible causes for non-ohmic contact behavior are mismatching of the work functions of the photoconductor and the electrode material, surface absorbed materials which form barrier layers or intergranular second phases which form internal barrier layers.

Subsequent to a sintering process as described in the above-mentioned US. application S.N. 172,699 filed February 12, 1962, now Patent No. 3,145,120, issued August 18, 1964, entitled, Method for Controlling Flux Pressure During a Sintering Process, by George Cheroff, Frederick Hochberg, Arnold Reisman, and S01 Triebwasser, the sintered photoconductor layers are washed in triple distilled water and are then etched 0.7 N-S N -HNO solution for 2 minutes to 30 minutes. They are dried for 10 minutes at 125 C. in air or in a vacuum desiccator for 2 hours at room temperature.

The photoconductor elements having electrodes afiixed thereto which are ohmic in nature are used in machine logic of computer mechanisms.

EXAMPLE 1 A sintered cadmium selenide activated photoconductor element containing parts per million of Cu is washed in triple distilled water until the Wash water fails to reveal any traces of cadmium chloride flux used in the sintering process. The photoconductor is then treated with a 0.7 N HNO solution for 30 minutes at room temperature and washed six times in triple distilled water. The photoconductor is then dried at C. in air for 10 minutes and cooled to room temperature. Silver paste electrodes are affixed by a standard silk screening process and these electrodes are dried in air at 165 C. for 2 hours. Electrical measurements of the finished electroded sintered layer show the linear current-voltage characteristics as depicted in curve A of FIGURE 1.

EXAMPLE 2 A sintered cadmium selenide activated photoconductor element containing approximately 100 parts per million of Cu is washed in triple distilled water until the wash fails to reveal any traces of cadmium chloride flux used in the sintering process. The photoconductor is then dried at 125 C. in air for 10 minutes and cooled to room temperature. Silver paste electrodes are aflixed by a silk screening process and these electrodes are dried at 165 C. for 2 hours in air. Electrical measurements of the finished electroded sintered layer photoconductor show non-linear current-voltage characteristics as shown in curve B of FIGURE 1.

EXAMPLE 3 The process of Example 1 is repeated except that 3 N HNO is used for 10 minutes instead of 0.7 N HNO for 30 minutes. The results are identical to those of curve A of FIGURE 1 (i.e. linear).

EXAMPLE 4 The process of Example 1 is repeated except that 5 N HNO is used for 2 minutes instead of 0.7 N HNO for 30 minutes. The results are identical to curve A of FIGURE 1 (i.e. linear).

EXAMPLE 5 The process of Examples 1, 3 or 4 is repeated except that after the final distilled water rinsing the sintered photoconductors are dried in a vacuum desiccator for 2 hours at room temperature under a pressure of 13 mm. of Hg. The results are identical to those of curve A of FIGURE 1 (i.e. linear).

EXAMPLE 6 Photoconductors are prepared as in Example 2 with an electrode separation of 10 mils and an electrode length of 150 mils exhibit an average resistance in the light of 3 kilohms under a 1000 microwatt per square centimeter incident neon light source. Other photoconductors prepared in the same manner, but with electrode separations of 5 mils and electrode length of 150 mils show an average light resistance of 2.2 kilohms. Both sets of photoconductors show non-linear I-V characteristics. If the scaling phenomenon had occurred the average light resistance of the second set would be 1.5 kilohms.

EXAMPLE 7 A set of 438 photoconductors are prepared which have a 30 mil electrode separation and 120 mils long according to Example 2. They exhibit a distribution of light resistance value shown in FIGURE 2, Curve I. Another set of 259 photoconductors having the same electrode geometry is prepared according to the process of Example 1. They exhibit a distribution of light resistance value shown in FIGURE 2, Curve II.

It is seen that reproducibility shown in FIGURE 2, Curve II is superior to that shown in FIGURE 2, Curve I and that the average sensitivity of the photoconductors prepared by the process of the invention as illustrated in Example 1 is approximately three times greater than those of the photoconductors prepared by the process of Example 2.

The process of the invention enables the preparation of photoconductors which exhibit linear current-voltage characteristics which are reproducible and which exhibit the phenomenon of electrical characteristic scaling. In accordance with the invention this is accomplished by making the surface receptive to the affixing of contacts through treatment of processed (sintered) photoconductors in 0.7 N to 5 N HNO for 2 to 30 minutes at room temperature.

While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

1. A process for preparing the surface layer of a cadmium selenide photoconductor element so that electrodes afiixed thereto define an ohmic contact between the surface of the electrode and the photoconductor element which comprises washing the photoconductor element with distilled Water to remove excess cadmium chloride flux used in said sintering process; treating the washed photoconductor element with nitric acid; washing the nitric acid treated photoconductor element with distilled water; and then drying the photoconductor element.

2. A process for electrically integrating electrodes to photoconductor elements which comprises treating the photoconductor element with nitric acid; affixing electrodes to the treated photoconductor element by a screening process to produce thereby photoconductor elements having electrodes which are ohmic in nature.

3. A process for affixing electrodes to cadmium selenide photoconductor elements which comprises treating the photoconductor element with nitric acid and afiixing silver paste electrodes to the treated photoconductor element by a screening process to produce thereby cadmium selenide photoconductor elements having silver paste electrodes which are ohmic in nature.

4. A process for atfixing electrodes to photoconductor elements which comprises washing the photoconductor element in distilled water until the wash water fails to reveal any traces of the flux used in the sintering process; treating the photoconductor element with 0.7 N to 5 N HNO solution for 2 to 30 minutes at room temperature; washing the thus treated photoconductor element in triple distilled water; drying the photoconductor element for 10 minutes at C. in air and cooling to room temperature; affixing electrodes by screening to the surface of the photoconductor element and drying this assembly in air at C. for 2 hours to produce thereby a photoconductor element having ohmic electrodes.

5. A process for afiixing silver paste electrodes to cadmium selenide photoconductor elements which comprises washing cadmium selenide photoconductor elements in triple distilled water until the wash water fails to reveal traces of cadmium chloride flux used in the sintering process; treating the photoconductor element with 0.7 N to 5 N HNO solution for 2 to 30 minutes at room temperature; washing the nitric acid treated photoconductor element with triple distilled water; drying the photoconductor element at 125 C. in air for 10 minutes and cooling to room temperature; afiixing silver paste electrodes by screening to the surface of the photoconductor element and drying this assembly in air at 165 C. for 2 hours to produce thereby a photoconductor element having an ohmic contact between the surface of the electrode and the photoconductor element.

6. A process for aflixing silver paste electrodes to a cadmium selenide photoconductor element which comprises:

(a) washing the cadmium selenide photoconductor element in triple distilled water until the wash water fails to reveal any traces of CdCl fiux used in the sintering step of the process of preparation;

(b) treating the washed photoconductor element with 5 0.7 N HNO for 30 minutes at room temperature;

(c) rewashing the HNO treated photoconductor element in triple distilled water six times;

((1) drying the photoeonductor element at 125 C. for

10 minutes and cooling to room temperature;

(e) affixing silver paste electrodes by a screening process and drying in air at 165 C. for 2 hours to produce thereby electrodes which are ohmic in nature;

7. In the process for preparing sintered layer photoconductor elements having electrodes aflixed thereto, the improvement which comprises treating the photoconductor element with HNO prior to affixing the electrodes.

References Cited by the Examiner FOREIGN PATENTS 9/1958 Canada. 3/1959 Canada. 5/ 1961 Herezog. 1/ 1945 Johnson.

RICHARD D. NEVIUS, Primary Examiner. 

2. A PROCESS FOR ELECTRICALLY INTEGRATING ELECTRODES TO PHOTOCONDUCTOR ELEMENTS WHICH COMPRISES TREATING THE PHOTOCONDUCTOR ELEMENT WITH NITRIC ACID; AFFIXING ELECTRODES TO THE TREATED PHOTOCONDUCTOR ELEMENT BY A SCREENING PROCESS TO PRODUCE THEREBY PHOTOCONDUCTOR ELEMENTS HAVING ELECTRODES WHICH ARE OHMIC IN NATURE. 